1. Field of the Invention
The invention relates to optical beam separators, particularly dichroic beam separators which separate the desired second or third harmonic beam from the residual fundamental beam.
2. Description of the Prior Art
A need exists in the laser community for a low-loss, highly damage-resistant, dichroic beam separator (DBS) for the purpose of separating the desired second or third harmonic beam from the residual fundamental beam. There are many situations where a designer might want to reject most of the residual power at the fundamental frequency from a laser operating at a harmonic frequency. For instance, this rejection is necessary in any operational scenario where such radiation must be prevented from striking either the designer's own detector or a remote one. The presence of any significant power at the fundamental will likely preclude any additional system operation at that wavelength by swamping the detector. For example, a pointing and tracking system using a Nd:YAG illuminator laser, which has a fundamental frequency of 1.06 .mu.m and which operates at the second harmonic, would be unable to detect missile plume emission in the region of 1.06 .mu.m unless the fundamental was suppressed because the detector would be swamped by the fundamental output of the laser.
The presence of any significant power at the fundamental may also compromise the covertness of an illuminator or range finder operating at a harmonic. For example, a tank, missile, or any other piece of enemy hardware may have tipoff detectors to determine whether the hardware is being illuminated. However, if these detectors operate at the fundamental bands of common lasers, they could not detect the output of a harmonically converted laser whose fundamental output had been suppressed
No other devices appear to achieve the minimum optical damage threshold for dichroic separation required by some high-powered laser systems. Referring to FIG. 1, the prior art discloses that a beam 2 exiting laser 4 is separated into a residual fundamental beam 6 and a frequency doubled beam 8 using a thin film dichroic coating 10 deposited on a glass substrate 12. The coating 10 is designed to reflect the harmonic beam and to transmit the fundamental beam. The fundamental beam would then be directed to an absorbing beam dump.
However, there are some shortcomings with this approach. For example, the use of a reflective element sharply changes the optical axis of the system and may limit design flexibility. More importantly, this approach is susceptible to optical damage when used in conjunction with high peak power lasers which operate at power densities in the range of 10.sup.7 to 10.sup.9 Watts/sq cm. An alternative to a reflective coating is a transmissive coating. Unfortunately, the transmissive coating is particularly vulnerable to damage from high power lasers because of the greater number of layers associated with this design.
Another prior art approach is the variable ratio beam splitter (VRB) disclosed by Durell in U.S. Pat. No. 4,787,710. The VRB is intended to split a monochromatic beam into two separate beams visible and infrared with a variable intensity ratio. However, the VRB is not able to pass one wavelength while rejecting another. Moreover, the VRB also involves a number of optical elements. This increases the size and weight of the apparatus, as well as the potential for misalignment. In addition, the VRB relies on coated optics which would be damaged by high power lasers.
Therefore, it is the object of the present invention to provide a single element, transmissive device that efficiently separates the fundamental from the harmonics in a beam, while exhibiting a high threshold for optical damage.